The present invention relates to digital imaging systems. More particularly, the present invention relates to the art of detecting image statistics.
In digital imaging devices such as digital cameras, a scene is captured by using a lens to direct light from the scene on an array of sensors, or photo-detectors, such as an array of photodiodes. Each sensor of the sensor array detects light from a tiny portion of the scene. At each sensor, the detected light is converted into an electrical signal, and then into a digital value indicating the intensity of the light detected by that sensor. Then, the digital values from all of the sensors of the sensor array are combined to form an image of the scene. For this reason, the sensor array is referred to as the image sensor array.
FIG. 1 illustrates a single sensor 10 including an active area 12 and a support hardware area 14. As illustrated, the active area 12 can be fabricated generally in the shape of a cross and the support hardware area 14 generally having a rectangular shape and adjoining the active area 12. The active area 12 includes a photo-detector to capture light to produce a corresponding electrical signal. The electrical signal is processed by support hardware in the support hardware area 14. The support hardware can include, for example, amplifiers, buffers, and converters. Light falling on the support hardware area 14 portion of the sensor 10 is lost as there are no photo-detectors in that area. For this reason, the support hardware area 15 is designed to occupy smallest area possible and covers smaller area than the area occupied by its active area 12. Additionally, the support hardware area 14 adjoins the active area 12 at a predetermined direction 17 relative to the active area 12. In the illustrated sensor 10, the support hardware area 14 adjoins the active area 12 at upper right direction relative to the active area 12.
Sizes of the sensor 10 and its component areas can vary greatly depending on the materials for the sensor 10, processes used to fabricate the sensor 10, and the desired application. For the sample sensor 10 of FIG. 1, the active area 10 can have a lateral extent 13 in the order of microns or tens of microns, for example, approximately 60 microns. The hardware support area 14 of the sensor 10 can have a lateral extent 15 in the order of microns or tens of microns, for example, approximately 20 microns.
Popular image sensor arrays include CMOS (complementary metal-oxide semiconductor) sensors and CCDs (charge-coupled devices) sensors. An image sensor array often includes a rectangular layout of many hundreds of thousands, millions, or even greater number of sensors, each sensor providing a digital value, or a pixel, of information. For example, a rectangular image sensor array arranged in 640 columns and 480 rows has 307,200 sensors, or pixels. A digital value from a sensor is defined as a pixel of the image. For convenience, terms “sensor” and “pixel” are herein used interchangeably unless otherwise noted, and each sensor, or pixel, is referred generically as Pi,j where i,j indicates that the pixel is located at ith column at jth row of a rectangular image sensor array having M columns and N rows, the value of i ranging from 1 to M, inclusive, and the value of j ranging from 1 to N, inclusive.
FIG. 2 illustrates a sample sensor array 20 including a rectangular array of sensors, each sensor having the configuration of the sensor 10 of FIG. 1. For simplicity, the sensor array 20 includes 36 sensors in a six by six rectangular configuration. Each sensor of the sensor array 20 having a cross shaped active area (for example, active area 22) adjoining a support hardware area (for example, support hardware area 24) to its upper right direction 26.
To capture a scene, each of the sensors of the image sensor array is initialized to an initial value. Then, the image sensor array is exposed to light from the scene for a period of time, the exposure period. After the exposure, the values of the sensors are read. The values from the sensors are combined to form an image of the scene. For best results, it is desirable to know various characteristics or statistics of the light from the scene to determine the operations of the image sensor array. For example, it is desirable to know the intensity of the light from the scene to determine the exposure period. For convenience, the characteristics or statistics of the light from the scene is referred to as “image statistics” in this document. For example, image statistics may include average pixel value.
Various approaches have been taken to gather image statistics during the image capture process. In a first approach a digital imaging device includes a reference detector positioned proximal to its image sensor array. Further, the reference detector is provided with its own lens and optical path through which light from the scene enters. In this design, the reference detector is operated substantially simultaneously as the image sensor array to collect image statistics. The reference detector can be a single detector or an array of detectors. Here, the cost, the complexity, and the bulk of the digital imaging device are increased and the reliability is decreased. This is because the reference detector needs its own lens and optical path, introducing additional components to the digital imaging device. Further, the image exposed to the reference detector is not identical to the image exposed to the image sensor array. Accordingly, the image statistics gather from the reference detector may only be marginally applicable to the image captured by the image sensor array, or the image statistics gather from the reference detector may not be applicable or even useful at all.
In a second approach, a beam splitter is used to split the light from a scene such that a first portion of the light from the scene is directed toward the image sensor array while a second portion of the light from the scene is directed toward a reference detector. Simultaneously, the reference detector senses the second portion of the light to gather images statistics. The image statistics can be used to determine the exposure period of the image sensor array or various processing parameters for processing of the captured image. Here, both the image sensor array and the reference detector see the identical scene. However, some of the light (the second portion) from the scene is lost to the image sensor because it is directed toward the reference detector.
In a third approach, the image sensor array is exposed (to the scene to be captured) twice in succession. During the first exposure, image statistics are gathered using the image sensor array. During the second exposure, the scene is captured as an image. In this design, no additional components are required, and the light from the scene is not apportioned. However, this approach requires two exposures of the image sensor array. Moreover, the lighting condition may have changed from the first exposure to the second exposure.
In a fourth approach, one or more sensors at the edges of the image sensor array are used as the reference detector to gather image statistics. Here, no additional components are required, and the light from the scene is not apportioned. However, the image statistics do not reflect the characteristics of the image near its center which is often the most interesting part of the image.
Accordingly, there remains a need for an improved digital imaging system for monitoring image statistics.